The functioning of the nervous system is dependent on the accurate establishment of synaptic circuits. The long-term goal of this work is to understand the mechanisms by which specific synaptic connections are formed, and the ways that these connections are adjusted during development. These questions are being addressed using a well-defined population of sensory neurons and interneurons in the cercal system of cockroach Periplaneta americana. The specific aims of this proposal are described below: There is a 20-fold increase in sensory input during the transition from first to second stage larva, a period of only 8 days. Our first aim is to determine the times of birth, axonal ingrowth and synapse formation of the new sensory neurons, an essential prerequisite for the later experiments. The second aim is to test the hypothesis that the Engrailed protein forms part of a system of positional information which determines the axon anatomy and synaptic connectivity of sensory neurons. The effects of antisense oligonucleotides and misexpression of Enrailed on the axonal arborizations and synaptic connections of developing sensory neurons will be assessed, using intracellular dye injection, electron microscopy and electrophysiology. The third aim is to test the role of cyclic AMP-dependent protein kinase (PKA), an antagonist of the Hedgehog signaling pathway, in determining axon arborization and synaptic connectivity. PKA will be activated or inhibited pharmacologically. The fourth aim is to investigate how the strengths of sensory neuron to interneuron synapses are adjusted as the animal grows and new inputs enter the nervous system. The experimental approaches include intracellular recording, quantal analysis, intracellular dye injection, and morphometric analysis. Many of the basic mechanisms which control the formation of neural circuitry appear to have been conserved during evolution, so the results derived from this study should be relevant to synapse formation during the development and regeneration of the human nervous system.